1. Field of the Invention
The invention relates to systems for determining the position of a point with respect to the earth, based on receiving radio-relay signals transmitted by a constellation of satellites orbiting the earth.
2. Discussion of the Background
There are currently two global positioning systems for carrying out this position determination almost anywhere on the earth; these being respectively the GPS system ("Global Positionning System") and the GLONASS system ("Global Orbiting Navigation Satellite System").
Both use a network of satellites circling the earth, each satellite regularly transmitting coded radio signals which a receiver can receive in order to compute its exact position in terms of longitude, latitude and altitude, and in addition its velocity and the exact time. A complementary system termed RGIC ("Random Geostationary Integrity Channel") is also being put in place; it uses geostationary satellites which transmit signals towards regions of the globe over which they are located.
The GPS system transmits spread-spectrum radio signals on a carrier frequency L1 equal to 1575.42 MHz, as well as signals on an auxiliary carrier frequency L2 of 1227.6 MHz. Here, only the frequency L1 will be of interest, but the principles set out below can be carried over, should the need be felt, to the frequency L2.
The signal spectrum is spread by pseudo-random codes, that is to say the signal on the carrier frequency L1 is modulated by a repetitive pseudo-random binary sequence also referred to as a PRN code (standing for "Pseudo-Random Noise"); in practice, the system uses two categories of PRN codes, namely;
the C/A codes ("Coarse Acquisition") transmitted at a bit frequency of 1.023 MHz; they are 1023 bits long and the duration of a complete sequence is 1 millisecond; the C/A codes allow approximate position determination, PA1 and the P codes (standing for "Precise") transmitted at a frequency of 10.23 MHz and allowing a more accurate fix. PA1 The terrestrial frame chosen is not the same: the centre of the earth is not exactly identical to that chosen for GPS. The direction of North is not entirely the same. Hence, if a GLONASS position is required from a GPS receiver, a transposition based on a conversion table or conversion software is necessary. PA1 In the GLONASS system the navigation data are transmitted at 100 baud rather than 50 and the frequency of the pseudo-random code is 511 kHz. PA1 Moreover, the carrier frequency L1 is not unique. Each satellite transmits on a particular frequency L1 and it is this frequency which allows it to be identified. The pseudo-random code is the same for all the satellites. It does not serve in identifying the satellite but merely in extracting the signal from the noise (spectral unspreading) and in determining the time discrepancies for accurate measurement of distance between the receiver and the satellite. The band of frequencies L1 used by the complete GLONASS system is fairly far removed from that of the GPS system; it goes from around 1600 MHz to around 1615 MHz. The frequencies L2 are likewise multiple. PA1 a radio signals reception chain (10, 12, 14, 16, 18, 34, 38) comprising circuits for transposing the carrier frequencies received to several transposed frequencies differing according to the carrier frequency received, and at least one analogue/digital converter (38) for converting the signals thus transposed into a digital signal with several transposed carrier frequencies corresponding to several satellites received simultaneously and transmitting on different carriers, PA1 at least one digital signal processing channel which receives the digital signal with several transposed carrier frequencies, each channel comprising a random code phase- and transposed carrier phase- servocontrol loop, the servocontrol loop comprising on the one hand at least one digital phase control oscillator (108) controlled by a frequency set-point signal and a phase error signal and on the other hand a programmable local pseudo-random code generator (114), PA1 code selection means enabling a user to apply to the code generator a signal for selecting one code from several possible codes, the various codes corresponding to the various satellites of the two networks, PA1 frequency selection means, enabling the user to apply to the oscillator one from several possible frequency set-point signals, the various set-point signals corresponding to the various satellites of the second network as well as to all of the satellites of the first network; PA1 the receiver finally comprising means (50, 80) for computing position from digital values provided in the servocontrol loop. PA1 receiving the signals from several satellites on an antenna, PA1 transposing the carrier frequency of the various signals received, thereby producing several transposed carrier frequencies differing according to the carrier received, PA1 applying several simultaneously received carrier frequencies to at least one common analogue/digital converter and converting the corresponding composite signal into a digital signal; PA1 applying the digital signals from the converter to at least one signal processing channel common to all the transposed carrier frequencies received from the converter; PA1 selecting a set-point frequency corresponding to a particular transposed frequency, from among several possible set-points corresponding to different satellites, and applying a corresponding set-point signal to a digital phase control oscillator so as to make this oscillator produce the set-point frequency, the oscillator moreover receiving a phase error signal derived by a phase servocontrol loop in which it is located; PA1 selecting a pseudo-random code and applying a corresponding set-point signal to a programmable pseudo-random local code generator so as to make the generator produce one desired code from several possible codes, the code generator being located in the servocontrol loop and correlation means being provided in order to shift the code produced so as to place it in synchronism with the identical code present in the modulation of the signal received from the converter; PA1 computing a receiver position from digital values provided in the servocontrol loop.
The logic transitions of the code are synchronized with the phase of the carrier L1, and the modulation is a phase modulation of a well defined type (BPSK).
The C/A codes are accessible to the public and must be generated locally within the receivers for three reasons: firstly, they make it possible to detect and demodulate the signals received, these signals being embedded in a very high level of noise (20 to 30 dB above the signal); detection is carried out by correlation between the code received from the satellite and an identical code produced locally; additionally, they make it possible to identify the transmitting satellite (in the GPS system a particular C/A code is assigned to each satellite); and lastly, they make it possible to measure time lags which are the indispensable basis for accurate computation of position.
The P codes are not accessible to the public; they are reserved for essentially military uses, and may moreover be encrypted on transmission.
The signals transmitted on the carrier frequency L1 are moreover coded using slow rate (50 baud) binary data which represent satellite navigation information, that is to say data serving in the position computations performed in the receiver. These navigation data are ephemerides which make it possible on the one hand to compute the exact position (accurate to within a meter) occupied by a satellite at any instant, and on the other hand to furnish all the receivers using the system with a common time reference. Here again, the modulation of the carrier L1 by the data is a phase modulation.
The distance between a satellite and the receiver is determined by measuring the duration of propagation, at the speed of light, of the radio signal between a satellite and the earth. It is therefore determined in particular by measuring the time discrepancy between the instant of transmission, by the satellite, of a characteristic bit (the "epoch" bit) of the pseudo-random code and the instant of reception by the receiver, of this characteristic bit.
The distances from the receiver to three different satellites make it possible to determine the position of the receiver in a fixed terrestrial frame once the receiver knows the position of the satellites in this frame at the instant of measurement. A fourth satellite makes it possible to eliminate the discrepancy between the clock of the receiver and the clock of the GPS system: the position of the satellites at each instant is defined by reference to a general system clock and the navigation data transmitted enable the receiver to ascertain this time reference. The measurement is made in two or three iterations on account of the fact that at the start, not possessing the exact time reference, it is only possible to lock on to the clock transmitted by the satellites to within 10 or 20 ms approximately, due to the signal propagation time from the satellites to the earth.
The relative speed of the receiver with respect to the satellites can also be determined by measuring the Doppler effect on the carrier frequency L1 received. The velocity of the receiver in the fixed frame of the terrestrial geoid is deduced from this.
The RGIC system is very similar to the GPS system. The GPS system uses circling satellites; the RGIC system uses geostationary satellites; it supplements the GPS system in order to cater for the latter's inadequacies or the risks of it breaking down. The signal types are still the same and the carrier frequency L1 is the same. Pseudo-random codes of the same length and frequency are used to identify the satellites; they differ from the codes of GPS satellites. The frequency of the navigation data is 250 baud instead of 50 baud. The data are coded by a Viterbi algorithm which carries out compression of the information.
The GLONASS system operates on similar general principles but significant differences may be noted:
Under these conditions, it is appreciated that a receiver designed to receive GPS signals is not suitable for computing a position from radio signals sent by GLONASS satellites, and vice versa.
There is however a considerable need to be able to use either system and the complementary systems such as RGIC also. This is because if certain satellites of one system are not correctly maintained, or if political reasons make them temporarily unusable, the consequences may be very serious for the users who will have established their operating procedures on the basis of one or other system. This is all the more important nowadays since the security of numerous systems is beginning to be based precisely on position detection by satellite. Such is the case in particular for maritime navigation and aerial navigation which are relying more and more on the GPS and GLONASS systems. For example, there are currently moves towards systematizing the landing-aid methods based on using signals from existing satellite networks.
It is of course always possible to employ two special-purpose receivers and to use one when the other cannot be used. This doubles the cost to the user. Or again, mixed receivers can be built which comprise a dual reception system. However, such a receiver is expensive since the number of GPS channels required is equal to the number of GPS satellites which it is desired to receive at the same time (at least four), and the number of GLONASS channels required is equal to the number of GLONASS satellites which it is desired to receive at the same time (at least four).
There is therefore a need for a cheap receiver which could receive GPS or GLONASS or RGIC signals or even other future systems, and which would not consist of a simple (expensive) juxtaposition of two different types of receivers.